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jspilot said:you are reducing the engine power which in turn reduces the air flow through the intake which increases manifold pressure....slower velocity - higher pressure......well at least it sounds good!!
flyifrvfr said:Can someone please tell me why manifold pressure increases when you decrease RPM'S when you are doing a run-up
UnAnswerd said:It doesn't actually increase, technically. You see, anyitme the engine is running, the pressure within the manifold will be lower than atmospheric pressure. We tend to think of it as a "vacuum". But "vacuum" isn't really the best term, because the pressure inside the manifold is not negative, it's just lower than atmospheric pressure.
When you close the throttle, the downward moving pistons are pulling this "vacuum" against a restriction. The actual pressure in the manifold decreases, but the pressure differential or so-called "vacuum" within the manifold increases.
Yellow Snow said:Keep trying you're closing in on it. Think about turbo charged engine though might have to rethink this.
UnstableAviator said:When you reduce the engine RPM, you reduce the engine's demand of air. Since the throttle position hasn't changed, the air will in effect "back up" in the intake tract thereby causing a pressure increase. It is just like Bernoulli's principle in reverse to the way we commonly think of it; Slower moving air exerts a higher pressure.
There is no back-up of air, the engine is actually increasing its power output and taking in more air.
Yellow Snow said:.......definately not how it was explained to me.
avbug said:A Squared is correct...and the rest of you had better do some more studying. Ouch.
Think of your engine as a vacum cleaner....it's a suction machine. What happens when you put your hand over the end of the hose on a vacum cleaner? It starts to scream and whine, and the pressure inside the hose goes down. Same suction, but you've blocked it off and as it's still sucking, the pressure drops.
This has nothing to do with bernoulli or airflow velocity vs. pressure. The engine is sucking, you're blocking it with the throttle plate, and pressure drops.
As A Squared noted, if you keep your throttle position constant and decrease the RPM's with the propeller control, you're slowing down the vacum cleaner...you're slowing down the engine, and it's producing less suction. If you're at a given throttle setting, you're not moving the throttle plate, so as the engine slows, there is less "suction" on the manifold beteen the throttle plate and each cylinder...therefore manifold pressure rises.
Pull the mixture to idle now, and you'll see a further rise in manifold pressure...right to current barometric pressure. If you're at sea level on a standard day, you'll get 29.92 inches of manifold pressure with the mixture at cutoff and the engine no longer turning. In a normally aspirated engine, the most manifold pressure you're going to get with the throttle wide open is barometric. If you're at sea level, then you're looking at nearly thirty inches, but if you're at five thousand feet you're looking closer to twenty five inches manifold pressure max.
If you want more than that, then you need turbocharging or another form of induction boosting.
Slowing the engine down with the propeller control during runup isn't boosting anything or adding power. It's reducing engine RPM and subsequently power, but showing an increase in manifold pressure because the engine is producing less "suction" at lower RPM's in the induction manifold between the throttle plate and the cylinders.
avbug said:I'm pretty busy with little time to answer, but you're incorrect.
Clamp your hand over a vacum cleaner, note the absolute pressure in the hose. Lift your hand off, note hte pressure. It rises. Same as opening the throttle. Lift your hand partially off, note an intermediate pressure.
Put your hand back over the hose, same as closing the throttle. Note the pressure. Same as before. Now slow down the speed of the vacum impeller. Same as decreasing engine RPM. Less suction, higher manifold pressure.
It has nothing to do with airflow velocity through the induction system and an attendant pressure rise or drop.